Electrical surge protector with thermal disconnect

ABSTRACT

A surge protection device comprising surge protector having a first terminal and a second terminal, one of the terminals being conductively connected to a first power transfer mechanism, such as a source of electrical power, and another of the terminals being conductively connected to a second power transfer mechanism, such as a sink of electrical power. The surge protector is operable to conduct current form one of the power transfer mechanism to another of the power transfer mechanism in the presence of a prolonged overvoltage surge across the first and second terminals of the surge protector. The surge protection device includes a mechanism for automatically disconnecting one of the terminals from one of the power transfer mechanisms in the presence of a prolonged overvoltage surge across the first and second terminals.

This is a continuation of U.S. Ser. No. 08/618,346 filed Mar. 16, 1996,now abandoned.

BACKGROUND OF THE INVENTION

The invention is directed to an electrical surge protector having amechanism for disconnecting the surge protector in response to aprolonged overvoltage surge and/or excessive surge currents exceedingthe rating of the surge protector.

Surge protection devices are used to protect components and powersystems from prolonged overvoltage surges, such as those caused bylightning for example. During a prolonged overvoltage surge, a surgeprotection device provides temporary surge protection of a component byshunting the overvoltage surge to ground or neutral. A prolongedovervoltage surge may cause a surge protector to overheat, thuspresenting a fire hazard.

SUMMARY OF THE INVENTION

The invention is directed to a surge protection device comprising asurge protector having a first terminal and a second terminal, one ofthe terminals being conductively connected to a first power transfermeans, such as a source of electrical power, and another of theterminals being conductively connected to a second power transfer means,such as a sink of electrical power. The surge protector is operable toconduct current from one of the power transfer means to another of thepower transfer means in the presence of a prolonged overvoltage surgeacross the terminals of the surge protector. The surge protection deviceincludes means for automatically electrically disconnecting one of theterminals from one of the power transfer means in the presence of aprolonged overvoltage surge across the surge protector terminals.

The automatic disconnecting means comprises a conductive member having afirst end with a fixed position and a second movable end. The first endis conductively coupled to one of the first and second power transfermeans, and the conductive member is flexible between a flexed positionin which the second end of the conductive member is positionedrelatively close to one of the surge protector terminals and an unflexedposition in which the second end of the conductive member is positionedrelatively far from the one surge protector terminal. The automaticdisconnecting means also includes thermally active means for holding theconductive member in the flexed position in the absence of a prolongedovervoltage surge.

The source of electrical power may be a power line of a multi-phasepower line, and the sink of electrical power may be electrical ground ora neutral conductor of the multi-phase power line. The surge protectiondevice may be provided as a surge protector module having a plurality ofsurge protectors disposed therein, each of the surge protectors havingmeans for automatically disconnecting one of the surge protectorterminals from one of the power transfer means in the presence of aprolonged overvoltage surge across the terminals of the surge protector.

These and other features and advantages of the present invention will beapparent to those of ordinary skill in the art in view of the detaileddescription of the preferred embodiment, which is made with reference tothe drawings, a brief description of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the display panel of a preferred embodiment of asurge protection system;

FIG. 2 is a side elevational view of a surge protector module havingfour surge protection devices;

Fig. 3 is a front elevational view of the surge protector module of FIG.2;

FIGS. 4A and 4B illustrate the operation of a thermal disconnect springassociated with one of the surge protection devices;

FIG. 4C is an enlarged view of a portion of the substrate 44 showing theconductor 54;

FIG. 5 is a circuit diagram of the surge protector module of FIGS. 2 and3;

FIG. 6 is a block diagram of the electronics of the surge protectionsystem;

FIG. 7 is a flowchart of the main operating routine of the surgeprotection system;

FIG. 8 is a flowchart of a check frequency routine shown schematicallyin FIG. 7;

FIG. 9 is a flowchart of a read voltages routine shown schematically inFIG. 7;

FIG. 10 is a flowchart of a check voltages routine shown schematicallyin FIG. 7; and

FIG. 11 is a flowchart of a determine protection remaining routine shownschematically in FIG. 7.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a preferred embodiment of a surge protection system10 designed to be connected to the individual power lines of amulti-phase power line, which is typically composed of two or threepower lines which have a common peak voltage of 120, 240, or 277V_(rms).

The surge protection system 10 has an outer housing 12 with a displayportion 14 that includes a display-select button 16, a numeric LEDdisplay 18, a line-select button 20, and a reset button 22. Thedisplay-select button 16 controls which of seven power conditions isdisplayed in the numeric display 18. The seven power conditions, each ofwhich has an LED 24 associated therewith, are shown in a display portion26.

Each time the display-select button 16 is pressed, the LED 24 associatedwith the next power condition is illuminated, and the numeric value ofthat power condition is shown in the numeric display 18. For example,when the display-select button 16 is pressed until the LED 24 associatedwith "Voltage" in the display portion 16 is illuminated, the numericvalue of the actual voltage on one of the lines of the multi-phase powerline is shown in the display 18. When the protection (%) condition issimilarly selected, the percent of the surge protection remaining forone of the lines is displayed.

The line-select button 20 allows the user to select one of the lines ofthe multi-phase power line for which the numeric value of the actualvoltage or percent protection remaining will be displayed. The lineswhich may be selected by the user, each of which has an LED 28associated therewith, are shown in a display portion 30. For example, todisplay the actual voltage on line B, the user would press thedisplay-select button 16 until the LED 24 associated with "Voltage" isilluminated, and then press the line-select button 20 until the LED 28associated with the B-N line is illuminated.

Referring to FIGS. 2 and 3, the surge protection system incorporates asurge protector module 40 for each phase or line of the multi-phasepower line. Each module 40 has four individual surge protection devices43, each of which incorporates a conventional surge protector 42 in theform of a metal-oxide varistor (MOV), attached to a carrier in the formof a printed circuit board 44 having four electrically conductiveconnection prongs 46.

Referring to FIG. 3, each surge protector 42 has a pair of electrodes inthe form of internal conductive rings 48 which are separated by aportion of metal oxide. The electrodes 48 and the metal oxide arecovered by an insulating coating. Each of the electrodes 48 iselectrically connected to one of a pair of terminals 50, 52, and theterminal 52 has a thermal disconnect mechanism attached to it in theform of a flexible conductor 54 and a portion of solder whichconductively interconnects the end of the conductor 54 with the terminal52. Other types of electrodes could be used.

As shown in FIGS. 3, 4A and 4C, the conductor 54 has a flat verticallydisposed portion 56 integrally formed with a C-shaped anchor portion 58(see FIG. 4C, which is an enlarged view of a portion of the substrate 44showing the conductor 54) which is disposed in a slot 60 formed in thesubstrate 44 to anchor the conductor 54 to the substrate 44. TheC-shaped portion 58 may be permanently fixed to the substrate 44 viasolder or a low-resistance conductive adhesive.

The surge protection device 43 of FIG. 4A is shown in an intact or readystate in which the terminal 52 is conductively coupled to the end of theconductor 54. In the event an overvoltage surge of sufficient magnitudeand duration occurs across the terminals 50, 52 of the surge protector42, the heat due to the relatively large current passing through themetal oxide portions of the surge protector 42 will melt the solderholding the end of the conductor 54 to the terminal 52, whereupon theend of the conductor 54 will be forced away from the terminal 52,causing the surge protection device 43 to be in a disconnected orinoperable state, as shown in FIG. 4B.

The conductor 54 may be composed of any suitable conductive materialthat remains flexible at relatively high temperatures, such as acopper-beryllium alloy (e.g. 0.2% beryllium). The flexible conductor 54may be provided with a predetermined spring bias or arc, or it maysimply be a straight metal portion, the end of which is flexed towardsthe surge protector terminal and held in place against the surgeprotector terminal by solder.

A circuit diagram of one of the surge protector modules 40 (which areidentical) is shown in FIG. 5. Referring to FIG. 5, the module 40 hastwo surge protectors 42 which have one terminal 50 connected to theneutral conductor of a multi-phase power line and the other terminal 52connected to one of the power lines (in this case line A) via theconductors 54 (shown as fuses 54). The other two surge protectors 42 inthe module 40 have one terminal 50 connected to a ground conductor andthe other terminal 52 connected to the same power line via theconductors 54. Four resistors 62 with identical resistance values eachhave a first end which is connected to the junction between theterminals 52 and the conductors 54 and a second end connected to avoltage-sensing line 64. A second voltage-sensing line 66 is utilized tosense the voltage on the power line to which the module 40 is connected.

When one (or more) of the surge protection devices 43 transitions fromthe ready state to the inoperable state, the fuse 54 associated withthat device 43 disconnects the associated surge protector 42 from thepower line. As a consequence of such transition, the magnitude of thevoltage on the line 64 changes.

In particular, it should be noted that the resistors 62 are connected inparallel (when the fuses 54 are intact) since the first end of eachresistor 62 is connected to a first common point (the line 64) and thesecond end of each resistor 62 is connected to a second common point(the line voltage). Thus, when all four fuses 54 are intact, thecombined resistance presented by all four resistors 62 is 1/4R, where Ris the resistance value of each resistor 62.

However, when a fuse 54 is disconnected, the resistor 52 associated withthat fuse 54 is effectively removed from the circuit, so that thecombined resistance of the remaining three resistors 52 is 1/3R.Similarly, when only two fuses 54 are intact, the combined resistance ofthe remaining two resistors 52 is 1/2R. When only one fuse 54 is intact,the resistance is R. Finally, when no fuses 54 are intact, theresistance is infinite.

Since the combined resistance of the remaining resistors 52 is inverselyrelated to the voltage on the line 64 (current flows through theresistors 62 and the line 64 to a half-wave rectifier circuit 106described below), the number of surge protection devices 43 which are inthe ready state can be determined based on the magnitude of the voltagesensed on the line 64, as described in more detail below.

A block diagram of the electronics of the surge protection system 10 isshown in FIG. 6. Referring to FIG. 6, the system 10 has amicrocontroller 80, such as a Motorola 6811 microcontroller, having amicroprocessor 82, a random-access memory (RAM) 84, a read-only memory(ROM) 86, an analog-to-digital (A/D) converter 88, an I/O interface 90,and an RS-232 interface 92, all of which are interconnected by anaddress/data bus 94.

The I/O interface 90 is connected to a display driver circuit 100 via amulti-signal line 102 and receives electronic input from a user (fromthe display-select button 16, the line-select button 20 and the resetbutton 22) via a multi-signal line 104. The RS-232 interface 92 may beused to conduct data communications with a device at a location remotefrom the surge protection system 10.

A half-wave rectifier circuit 106 is connected to the voltage-sensingline 64 of each surge protector module 40. Each rectifier 106, whichgenerates a half-wave rectified signal which is supplied to the A/Dconverter 88 via one of three respective lines 108a-108c, is in the formof a diode connected to three resistors in series (not shown). Each ofthe lines 64 is connected to the input of one of the diodes; the threeresistors of each rectifier 106 are connected to the neutral conductorof the multi-phase power line; and each of the lines 108a-108c isconnected to a respective junction between the diode and the resistorsof each rectifier 106.

A low-pass filter circuit 110 is connected to the voltage-sensing line66 of each surge protector module 40. Each filter 110, which may be aconventional low-pass filter circuit having an operational amplifier(not shown), is connected to the A/D converter 88 via one of threerespective lines 112a-112c.

The system 10 may also have a protector module 114 coupled between theneutral conductor of the multi-phase power line and ground. The neutralconductor protector module 114 may be composed of a 150 volt metal oxidevaristor and a 270 volt spark gap connected in parallel between theneutral conductor and ground.

OVERALL OPERATION

FIG. 7 illustrates a flowchart of a main operating routine 200 of acomputer program stored in the ROM 86 which is executed by themicroprocessor 82 to control the operation of the system 10. Referringto FIG. 7, the first task that is performed by the system 10, whichoccurs at step 220, is to check the frequency of the alternating current(AC) waveforms on the multi-phase power line to which the system 10 isattached, which frequency is either 50 or 60 Hz. After the frequency isdetermined at step 220, the remaining portion of the routine 200 isperformed approximately synchronously with the frequency. Thus, wherethe frequency is 60 Hz, steps 222-250 would be repeated once every 16.7milliseconds.

At step 222, the root-mean-square (RMS) value of the voltages sensed onthe three power lines are calculated, based on a read voltages routinethat periodically reads the voltages supplied to the A/D converter 88via the lines 112a-112c, as described below.

At step 230, the three voltages calculated at step 222 (one voltage foreach power line) are checked for various conditions, such as whether thevoltages are above or below their expected nominal levels.

At step 232, if there was a power outage on all three power lines (ifthe outage count of step 342 of FIG. 10 equals three), the programbranches to step 234 where data regarding the various voltage conditionsis stored in a nonvolatile memory (not shown) so that the data is notlost due to power loss to the system 10.

At step 236, if the system 10 detects that the user pressed one of thebuttons 16, 20, 22, the system 10 responds appropriately to the userinput at step 238. At step 240, the display 14 is updated by changing(if necessary) the numeric value of the voltage condition displayed inthe numeric display 18. At step 250, the system 10 determines the amountof surge protection that is remaining, based on the magnitudes of thevoltages sensed on the three lines 64, as described in more detailbelow.

Check Frequency Routine

FIG. 8 is a flowchart of a check frequency routine shown schematicallyin FIG. 7 as step 220. The theory of operation of the frequency checkroutine 220 is that if an AC waveform is sampled at the same time duringeach periodic cycle (i.e. the sampling frequency is the same of thefrequency of the AC waveform), then all the samples should havesubstantially the same magnitude. The routine 220 determines whether allsamples are substantially the same by repeatedly determining thedifference in magnitude between successive samples, summing thosedifferences, and determining whether the sum of the differences of allthe samples falls below a threshold limit. If it does, then the samplingrate is assumed to correspond with the frequency of the AC waveform.

Referring to FIG. 8, the frequency check routine 220 begins operation atstep 260 by setting a sample count to zero (64 samples of the voltage online A are taken). At step 262, a sum variable is set to zero, and atstep 264 the sampling rate is initially set to correspond to 60 Hz. Inthis case, the sampling rate is 60 Hz×64 samples per cycle, or 3.84 kHz.

Steps 266-274 are then repeated until all the voltage samples of line Aare processed. At step 266, the channel of the A/D converter 88connected to the voltage-sensing line 112a is read. At step 268, thecurrent voltage magnitude (i.e. the voltage magnitude just read from theA/D converter 88 at step 266) is subtracted from the previous voltagemagnitude to determine the difference between the successive voltagereadings. At step 270, that difference is added to the sum variable(which was set to zero at step 262), and at step 272, the sample countis incremented. At step 274, if the sample count does not equal 64,meaning that not all samples have been taken, the program repeats steps266-274.

At step 276, if the accumulated sum of all magnitude differences betweenadjacent voltage samples is greater than a predetermined threshold, inwhich case it is assumed that the sampling frequency was not the same asthe frequency of the AC waveform on line A of the multi-phase powerline, then the program branches to step 278 where the sampling rate ischanged to correspond to a 50 Hz waveform (which is the only otherfrequency option besides 60 Hz). In this case, the sampling rate is 50Hz×64 samples per cycle, or 3.2 kHz. Otherwise, the sampling rate ismaintained to correspond with a 60 Hz waveform.

Read Voltages Routine

FIG. 9 is a flowchart of a read voltages routine 300 which periodicallyreads the voltages provided to the A/D converter 88 via thevoltage-sensing lines 108, 112. The routine 300 may be an interruptservice routine, the performance of which is periodically triggered byan interrupt signal generated by a clock circuit, for example. Theroutine 300 takes 64 voltage samples for each cycle of the AC waveformson the voltage-sensing lines 112a-112c (where the frequency of the ACwaveforms is 60 Hz, the routine 300 is performed approximately every 260microseconds) and one voltage sample for each cycle of the half-waverectified waveforms on the voltage-sensing lines 108a-108c.

Referring to FIG. 9, at step 302, if the sample count (which was set to64 in step 318 during the prior execution of the routine 300) is notzero, meaning that another voltage sample of one of the lines 112a-112cshould be taken, the program branches to step 304, where the samplecount is decremented by one.

At step 306 a first channel of the A/D converter 88 is read to obtain abinary sample of the voltage on one of the voltage-sensing lines112a-112c. At step 308, the program checks to determine whether themagnitude of the binary sample is zero by determining whether it iswithin a predetermined zero band defined by upper and lower binarylimits. For example, for an A/D converter that generates an 8-bit outputsignal having a maximum positive value of +128 and a maximum negativevalue of -127, the zero band could be defined to have an upper binarylimit of +2 and a lower binary limit of -2.

If the voltage sample falls within that zero band, it is assumed thatthe magnitude of the voltage sample is zero, in which case the programbranches to step 310 where a zero counter is incremented. The count ofthe zero counter is later used to detect the presence of a fractionalcycle dropout (i.e. an unexpected relatively short duration zerovoltage). A normal voltage signal having no fractional cycle dropoutsshould generate a relatively small number of zero counts since thecorresponding AC waveform passes through the zero axis twice for eachfull cycle. A zero count greater than that relatively small numberindicates the presence of one or more fractional cycle dropouts.

At step 312, the magnitude of the voltage sample taken at step 306 issquared and then added to an accumulating sum. Step 312 performs thefirst two steps necessary to calculate the RMS voltage, which is thesquare root of the sum: V₁ ² +V₂ ² +V₃ ² +V₄ ² . . . , where each V_(x)is the magnitude of one of a number of successive voltage samples (thefinal step in calculating the RMS voltage is step 222 of FIG. 7).

Steps 306-314 are performed once for each line, i.e. if the multi-phasepower line has three lines, the A/D converter 88 is read three times toobtain a single sample of each line voltage. At step 314, if all threeline voltage samples have been obtained, then the routine ends.

If the sample count was zero as determined at step 302, meaning that theroutine 300 has been performed 64 times and all 64 samples of thevoltage on the three lines 112a-112c have been taken, the programbranches to step 316 where the accumulated sum (the sum calculated instep 312 during the prior execution of the routine 300) for each of thethree line voltages is stored. At step 318, the sample count is set to64, and at step 320, the accumulated sum is reset to zero since a newset of 64 voltage samples will be started the next time the routine 300is initiated.

At step 322, the A/D converter 88 is read three times to obtain samplesof the half-rectified voltages on the voltage-sensing lines 108a-108c.It should be noted that step 322 is performed every 65th time theroutine 300 is performed (only after the sample count counts down from64 to zero) so that the three half-wave rectified waveforms on the lines108a-108c (which have the same frequency as the AC waveforms on thelines 112a-112c) are sampled slightly less than about once per cycle,and thus the sampling frequency is lower than the frequency of thehalf-wave rectified waveforms. The voltage samples taken at step 322 aresubsequently used to determine the amount of surge protection remaining,as described in more detail below.

Check Voltages Routine

FIG. 10 is a flowchart of a check voltages routine shown schematicallyin FIG. 7 as step 230. The check voltages routine 230 periodicallychecks the RMS voltage, calculated at step 222 of FIG. 7, for each lineof the three-phase power line to determine whether the voltages arewithin normal operating limits.

At step 330, an outage count is set to zero. The outage count keepstrack of the number of lines of the multi-phase power line for which apower outage was detected.

At step 332, a pointer is set to retrieve the data for the next (orfirst) line voltage. At step 334, if the zero count for that line isgreater than a predetermined limit, the program branches to step 336where a dropout flag is set, indicating the presence of a fractionalcycle dropout, as described above.

Step 338 determines whether the RMS voltage for that line (as calculatedat step 222 of FIG. 7) is less than a predetermined limit, such as 90%of the expected nominal RMS voltage, to detect the presence of a voltage"sag." If the voltage is less than the sag limit, the program branchesto step 340, where the voltage is compared with an outage limit, such as10% of the expected nominal voltage, to detect the presence of a poweroutage. If the voltage is less than the outage limit, the programbranches to step 342, where the outage count initialized to zero at step330 is incremented by one. If the voltage is not less than the outagelimit as determined at step 340 (but is less than the sag limit asdetermined at step 338), the program branches to step 344 where a sagflag is set to indicate the presence of a voltage sag.

Step 346 determines whether the RMS voltage for that line is greaterthan a predetermined limit, such as 110% of the expected nominalvoltage, to detect the presence of a voltage "swell." If the voltage isgreater than the swell limit, the program branches to step 348, where aswell flag is set to indicate the presence of a voltage swell.

At step 350, if the RMS voltages for all three lines of the power linehave not been checked, the program branches back to step 332. Otherwise,at step 352, if the dropout flag was set at step 336 pursuant to thedetection of a fractional cycle dropout in any one of the three lines, adropout display counter (which is a software counter) is incremented atstep 354. The count specified by the dropout display counter isdisplayed in the numeric display 18 when the select-display button 16 isused to illuminate the LED 24 associated with "Dropouts" in the displayportion 26 (see FIG. 1).

It should be noted that the dropout display counter is incremented atstep 354 if a fractional cycle dropout was detected in any of the threelines of the multi-phase power line. Consequently, when the user electsto have the number of dropouts displayed (via the display-select button16), the number of dropouts is the same for all three lines, and allfour LEDs 28 in the display portion 30 are illuminated (in this case,the line-select button 20 is ineffective since the display 18 relates toall lines).

At step 356, if the sag flag was set at step 344 pursuant to thedetection of a voltage sag in any one of the three lines, a sag displaycounter is incremented at step 358. The count specified by the sagdisplay counter is displayed in the numeric display 18 when theselect-display button 16 is used to illuminate the LED 24 associatedwith "Sags" in the display portion 26 (see FIG. 1). When voltage sagsare selected, all LEDs 28 in the display portion 30 are illuminatedsince the sags relate to all lines.

At step 360, if the swell flag was set at step 348 pursuant to thedetection of a voltage swell in any one of the three lines, a swelldisplay counter is incremented at step 362. The count specified by theswell display counter is displayed in the numeric display 18 when theselect-display button 16 is used to illuminate the LED 24 associatedwith "Swells" in the display portion 26. When voltage swells areselected, all LEDs 28 in the display portion 30 are illuminated sincethe swells relate to all lines.

At step 364, if the outage count (controlled by steps 330, 342) is notequal to zero, meaning that there was a power outage in at least one ofthe three lines, an outage display counter is incremented at step 366.That count is displayed in the numeric display 18 when theselect-display button 16 is used to illuminate the LED 24 associatedwith "Outages" in the display portion 26. When power outages areselected, all LEDs 28 in the display portion 30 are illuminated sincethe outages relate to all lines.

Determine Protection Remaining Routine

FIG. 11 is a flowchart of the determine protection remaining routineshown schematically as step 250 in FIG. 7. The routine 250 utilizes aweighted running average of the voltages generated on thevoltage-sensing lines 108a-108c to determine the amount of surgeprotection remaining in each of the surge protector modules 40.Referring to FIG. 11, steps 400-428 are performed once for each surgeprotector module 40 that protects a line of the multi-phase power line.

At step 400, a pointer is set to retrieve the voltage data for the surgeprotector module 40 that protects one of the lines of the multi-phasepower line. This data includes the most recent voltage for that line,which was read at step 322 of the read voltages routine 300 (FIG. 9),and the average voltage that was calculated during the previousperformance of step 402 of the routine 250.

At step 402, the weighted average voltage V_(ave) is determined inaccordance with the following equation:

    V.sub.ave =C.sub.1 V.sub.s +C.sub.2 i V.sub.ave-1,

where C₁ is a constant, such as 1/256, where V_(s) is the latest samplevoltage obtained during step 322 of FIG. 9, where C₂ is a constant, suchas 255/256, and where V_(ave-1) is the voltage average calculated duringthe previous execution of step 402.

As noted above, the rate at which the voltage samples V_(s) aregenerated is slightly different (slower) than the frequency of thehalf-wave rectified waveforms on the lines 108a-108c. The practicalconsequence of generating the V_(s) samples at a slightly different rateis that it does not matter where in the cycle of the rectified waveformthe sampling takes place. For example, if the sampling rate was the sameas the waveform frequency, then all samples would be taken at the samerelative place in the waveform, for example, at the peak of thewaveform. In that case, the system would require some type of means fordetermining the point in the waveform at which the samples were beingtaken.

However, by using a weighted average of voltage samples V_(s) taken at aslightly different frequency than the waveform frequency, the magnitudeof the weighted average over time is indicative of the number of surgeprotection devices 43 that are in the ready state, since the magnitudeof each V_(s) sample depends upon how many of the surge protectiondevices 43 in each module 40 are in the ready state.

At step 404, the voltage average V_(ave) computed at step 402 iscompared with a first, relatively high voltage threshold K₄. It shouldbe noted that, if all four surge protection devices 43 of the module 40are in the ready state, the four resistors 62 will have a relatively lowcombined resistance of 1/4R, and consequently the average voltage on theline 64 will be relatively high since there is only a relatively smallvoltage drop across the resistance 1/4R. If V_(ave) is greater than theK₄ threshold, it is assumed that all four surge protection devices 43are in their ready state, and thus that the surge protection remainingfor that module 40 is 100%, in which case the program branches to step406 where the protection remaining is set to 100%.

At step 408, the voltage average V_(ave) computed at step 402 iscompared with a second, somewhat lower voltage threshold K₃ (K₃ <K₄). Ifonly three of the four surge protection devices 43 of the module 40 arein the ready state, the three corresponding resistors 62 will have acombined resistance of 1/3R, and consequently the average voltage on theline 64 will be somewhat lower due to the larger voltage drop across thecombined resistance 1/3R. If V_(ave) is greater than the K₃ threshold(but less than K₄), it is assumed that three of the four surgeprotection devices 43 are in their ready state, and the program branchesto step 410 where the protection remaining is set to 75%.

At step 412, the voltage average V_(ave) computed at step 402 iscompared with a third, lower voltage threshold K₂ (K₂ <K₃). If only twoof the four surge protection devices 43 of the module 40 are in theready state, the two corresponding resistors 62 will have a combinedresistance of 1/2R, and consequently the average voltage on the line 64will be lower due to the larger voltage drop across the combinedresistance 1/2R. If V_(ave) is greater than the K₂ threshold (but lessthan K₃), it is assumed that only two of the four surge protectiondevices 43 are in their ready state, and the program branches to step414 where the protection remaining is set to 50%.

At step 416, the voltage average V_(ave) is compared with a fourthrelatively low voltage threshold K₁ (K₁ <K₂). If only one of the foursurge protection devices 43 is in the ready state, the single resistor62 will have a resistance of R, and consequently the average voltage onthe line 64 will be lower still. If V_(ave) is not greater than the K₁threshold as determined at step 416, it is assumed that none of the foursurge protection devices 43 is in its ready state, and the programbranches to step 420 where the protection remaining is set to 0%.

At step 422, the numeric display 18 is updated to reflect the amount ofsurge protection remaining. When the display-select button 16 is set toilluminate the LED 24 associated with "Protection %" in the displayportion 26, the percentage of surge protection remaining is shown in thenumeric display 18 for the surge protection module 40 specified by theline-select button 20.

At step 424, if the amount of surge protection just calculated is below75%, the program branches to step 426 where an audio alarm is generatedto warn the user of that fact. In addition to generating an audio alarmat step 426, an optional remote monitoring relay could be activated togenerate a warning signal. The monitoring relay could be provided with apredetermined activation delay to prevent false alarms due to relativelybrief power outages that would cause the monitoring relay to bedeenergized. At step 428, if the surge protection modules 40 for allthree lines of the power line have not been checked, the programbranches back to step 400, and steps 400-428 are repeated for the module40 protecting the next line.

It should be noted that the manner of determining the weighted averagein step 402 via the computer program could be implemented by using alow-pass filter connected to the output of each of the half-waverectifiers 106. It should further be noted that the comparisonsperformed at steps 404, 408, 412, and 416 could be implemented inhardware, instead of software, by using four comparators.

Numerous additional modifications and alternative embodiments of theinvention will be apparent to those skilled in the art in view of theforegoing description. This description is to be construed asillustrative only, and is for the purpose of teaching those skilled inthe art the best mode of carrying out the invention. The details of thestructure and method may be varied substantially without departing fromthe spirit of the invention, and the exclusive use of all modificationswhich come within the scope of the appended claims is reserved.

What is claimed is:
 1. An apparatus, comprising:a surge protector havinga first terminal and a second terminal, one of said terminals beingconductively connected to a first power transfer means and another ofsaid terminals being conductively connected to a second power transfermeans, said surge protector being operable to conduct current from oneof said power transfer means to another of said power transfer means inthe presence of an overvoltage surge across said first and secondterminals of said surge protector; and means for automaticallyelectrically disconnecting one of said terminals of said surge protectorfrom one of said first and second power transfer means in the presenceof a prolonged overvoltage surge across said terminals of said surgeprotector, said disconnecting means comprising:a conductive memberhaving a first end with a fixed position and a second movable end, saidfirst end being conductively coupled to one of said first and secondpower transfer means, said conductive member being flexible between aflexed position in which said second end of said conductive membercauses said one terminal of said surge protector to be conductivelyconnected to said one power transfer means and an unflexed position inwhich said second end of said conductive member causes said one terminalof said surge protector to be conductively disconnected from said onepower transfer means, said conductive member comprising a flat stripwith a rectangular cross-section having a first dimension and a seconddimension, said first dimension being at least twice as great as saidsecond dimension; and thermally active means for holding said conductivemember in said flexed position in the absence of a prolonged overvoltagesurge and for causing said conductive member to move to said unflexedposition in response to heating of said surge protector.
 2. An apparatusas defined in claim 1 wherein said second end of said conductive memberis in physical contact with said one terminal of said surge protectorwhen said conductive member is in said flexed position.
 3. An apparatusas defined in claim 1 wherein each of said power transfer meanscomprises an electrical connector.
 4. An apparatus as defined in claim 1wherein said first power transfer means comprises a conductive memberadapted to be connected to a source of electrical power and wherein saidsecond power transfer means comprises a conductive member adapted to beconnected to a sink of electrical power.
 5. An apparatus as defined inclaim 4 wherein said source of electrical power comprises a power lineof a multi-phase power line and wherein said sink of electrical powercomprises electrical ground.
 6. An apparatus as defined in claim 4wherein said source of electrical power comprises a power line of amulti-phase power line and wherein said sink of electrical powercomprises a neutral conductor of said multi-phase power line.
 7. Anapparatus as defined in claim 1 wherein said thermally active meanscomprises solder.
 8. An apparatus, comprising:a surge protector modulehaving a first surge protector and a second surge protector, said firstsurge protector having a first terminal and a second terminal, one ofsaid terminals being conductively connected to a first power transfermeans and another of said terminals being conductively connected to asecond power transfer means, said first surge protector being operableto conduct current from one of said power transfer means to another ofsaid power transfer means in the presence of an overvoltage surge acrosssaid terminals of said first surge protector, said second surgeprotector having a first terminal and a second terminal, one of saidterminals of said second surge protector being conductively connected toa third power transfer means and another of said terminals of saidsecond surge protector being conductively connected to a fourth powertransfer means, said second surge protector being operable to conductcurrent from one of said third and fourth power transfer means toanother of said third and fourth power transfer means in the presence ofan overvoltage surge across said second terminals of said second surgeprotector; first means for automatically electrically disconnecting oneof said terminals of said first surge protector from one of said firstand second power transfer means in the presence of a prolongedovervoltage surge across said terminals of said first surge protector,said first means comprising;a first conductive member having a first endwith a fixed position and a second movable end, said first end beingconductively coupled to one of said first and second power transfermeans said first conductive member being flexible between a flexedposition in which said second end of said first conductive member ispositioned relatively close to said one terminal of said first surgeprotector and an unflexed position in which said second end of saidfirst conductive member is positioned relatively far from said oneterminal of said first surge protector, said first conductive membercomprising a flat strip with a rectangular cross-section having a firstdimension and a second dimension, said first dimension being at leasttwice as great as said second dimension; and thermally active means forholding said first conductive member in said flexed position in theabsence of a prolonged overvoltage surge and for causing said firstconductive member to move to said unflexed position is response toheating of said first surge protector; and second means forautomatically electrically disconnecting one of said terminals of saidsecond surge protector from one of said third and fourth power transfermeans in the presence of a prolonged overvoltage surge across saidterminals of said second surge protector, said second means comprising;asecond conductible member having a first end with a fixed position and asecond movable end, said first end of said second conductive memberbeing conductively coupled to one of said third and fourth powertransfer means, said second conductive member being flexible between aflexed position in which said second end of said second conductivemember is positioned relatively close to said one terminal of saidsecond surge protector and an unflexed position in which said second endof said second conductive member is positioned relatively far from saidone terminal of said second surge protector, said second conductivemember comprising a flat strip with a rectangular cross-section having afirst dimension and a second dimension, said first dimension of saidsecond conductive member being at least twice as great as said seconddimension of said second conductive member; and thermally active meansfor holding said second conductive member in said flexed position in theabsence of a prolonged overvoltage surge and for causing said secondconductive member to move to said unflexed position in response toheating of said second surge protector.
 9. An apparatus as defined inclaim 8 wherein each of said first and third power transfer meanscomprises a conductive member adapted to be connected to a source ofelectrical power and wherein each of said second and fourth powertransfer means comprises a conductive member adapted to be connected toa sink of electrical power.
 10. An apparatus as defined in claim 9wherein each of said sources of electrical power comprises a power lineof a multi-phase power line and wherein each of said sinks of electricalpower comprises electrical ground.
 11. An apparatus as defined in claim9 wherein each of said sources of electrical power comprises a powerline of a multi-phase power line and wherein each of said sinks ofelectrical power comprises a neutral conductor of said multi-phase powerline.
 12. An apparatus as defined in claim 8 wherein each of saidthermally active means comprises solder.
 13. An apparatus, comprising:asurge protector having a first terminal and a second terminal, saidfirst terminal being conductively connected to a first power transfermeans and said second terminal being conductively connected to a secondpower transfer means, said surge protector being operable to conductcurrent from one of said power transfer means to another of said powertransfer means in the presence of an overvoltage surge across said firstand second terminals of said surge protector; and means forautomatically electrically disconnecting said first terminal of saidsurge protector from said first power transfer means in the presence ofa prolonged overvoltage surge across said terminals of said surgeprotector, said disconnecting means comprising:a conductive memberhaving a first end with a fixed position and a second movable end, saidfirst end being conductively coupled to said first power transfer means,said conductive member being flexible between a flexed position in whichsaid second end of said conductive member causes said first terminal ofsaid surge protector to be conductively connected to said first powertransfer means and an unflexed position in which said second end of saidconductive member causes said first terminal of said surge protector tobe conductively disconnected from said first power transfer means, saidconductive member comprising a flat strip with a rectangularcross-section having a first dimension and a second dimension, saidfirst dimension being at least twice as great as said second dimension;and meltable thermally active means for holding said conductive memberin said flexed position in the absence of a prolonged overvoltage surgeand for causing said conductive member to move to said unflexed positionin response to heating of said surge protector.
 14. An apparatus asdefined in claim 13 wherein said second end of said conductive member isin physical contact with said first terminal of said surge protectorwhen said conductive member is in said flexed position.
 15. An apparatusas defined in claim 13 wherein each of said power transfer meanscomprises an electrical connector.
 16. An apparatus as defined in claim13 wherein said first power transfer means comprises a conductive memberadapted to be connected to a source of electrical power and wherein saidsecond power transfer means comprises a conductive member adapted to beconnected to a sink of electrical power.
 17. An apparatus as defined inclaim 16 wherein said source of electrical power comprises a power lineof a multi-phase power line and wherein said sink of electrical powercomprises electrical ground.
 18. An apparatus as defined in claim 16wherein said source of electrical power comprises a power line of amulti-phase power line and wherein said sink of electrical powercomprises a neutral conductor of said multi-phase power line.
 19. Anapparatus as defined in claim 13 wherein said meltable thermally activemeans comprises solder.